The invention relates to the field of marine seismic exploration. More particularly, the invention relates to an improved system and method for connecting marine seismic streamer cables towed behind a vessel.
Seismic sources and acoustic energy sensing hydrophones are deployed behind seismic vessels to survey the subsurface geologic formations underlying water. The seismic sources produce acoustic energy propagated into subsurface geologic formations, and acoustic energy is reflected upwardly as acoustic waves detectable by the hydrophones.
Seismic streamers comprise hydrophones, electronic modules, electrical wires, and sensors. Seismic streamers are typically thousands of meters long and are positioned at selected cross offsets. Multiple streamers are towed in a seismic array through the water, and the outer streamers are offset from the vessel centerline with paravanes and other diverter systems. Lead-in cables connect the streamers to the tow vessel, and various techniques maintain constant spacing between adjacent streamers.
Tether points are linked as the lead-in cable leaves the tow vessel back deck and are reeled out together behind the tow vessel. During this reeling operation, attached paravanes progressively move the outer tether points further away from the tow vessel line of travel. Early in such deployment, the cables can become entangled and create a spaghetti type effect behind the tow vessel. This can occur as several kilometers of streamer cable flow in the water behind the tether point.
Two common methods are used in the deployment and tow of a marine streamer array. One conventional method uses rolling blocks or sheaves to travel down a divertor tow wire while attached to a streamer cable. Another method utilizes a cross-tag line to position the streamer cables at a predetermined spacing. The cross-tag lines are shackled from one cable to another during deployment and during production. If one cable needs to be retrieved, all of the other cross-tag shackled cables must be retrieved. This relationship creates problems when multiple cables are retrieved or deployed with narrow spacing therebetween.
Various tackle systems have been developed for marine two operations. U.S. Pat. No. 3,568,623 to Gustavson et al. (1971) disclosed a tow tackle having a hinged wedge attachable to a slotted skirt. U.S. Pat. No. 4,574,723 to Chiles et al. (1986) disclosed a hitch device for permitting horizontal deployment of a paravane before the paravane is rotated to the vertical operating orientation. U.S. Pat. No. 5,408,947 to Curto et al. (1995) disclosed a removable towing bracket having pivoting arms for connection with a streamer cable lead-in.
U.S. Pat. No. 6,074,253 to Brinchmann-Hansen (2000) disclosed a device for connecting a seismic streamer to a lead-in cable. The device established electrical connection between the streamer and lead-in cable while reducing drag induced by water resistance as the cables were towed through the water.
A paravane handling system was disclosed in U.S. Pat. No. 4,574,723 to Chiles et al. (1986), wherein a paravane and hitching device were tethered to a main line. A latching mechanism was slidable along the main line to selectively engage and disengage the hitching device.
A need exists for improved systems and techniques for deploying and recovering seismic streamers in a marine seismic array. The system should be economic and should minimize water resistance as the seismic equipment is towed through water.
The invention provides a system and method for deploying at least two seismic streamers behind a tow vessel movable in a selected course through water. The system comprises a first cable having a first end attachable to a first seismic streamer and having a second end engaged with the vessel, a second cable having a first end attachable to a second seismic streamer and having a second end engaged with the vessel, a latch associated with the first cable, a tag line having a first end and having a second end connected with the second cable, wherein the tag line first end is slidable along the first cable until the tag line first end contacts the latch, and a diverter connected to the first cable first end for urging the first seismic streamer to a position offset from the vessel course and for maintaining tension in the tag line for controlling the distance between the first seismic streamer and the second seismic streamer.
The method of the invention comprises the steps of attaching a divertor to a first cable having an end engaged with the tow vessel and having a latch attached to the first cable, of deploying the divertor and first cable into the water, of engaging the first end of a tag line to the first cable, wherein said tag line has a second end connected with a second cable, of deploying the tag line and second cable into the water, wherein the tag line first end is slidable along the first cable, of adjusting the length of the second cable until the tag line first end contacts the latch, and of moving the tow vessel to pull the divertor and first and second cables through the water.